1. Field of the Invention
The present invention is in the field of optics, specifically in changing the characteristics of the resonance of optical waveguide micro-resonators, very small optical micro-resonators with sizes on the order of 0.1 micrometer to 1 millimeter. Examples of such waveguide-based micro-resonators include, optical micro-ring resonators, and one-dimensionally periodic photonic band gap waveguide structures.
2. Prior Art
Micro-resonators, which are micrometer-sized optical resonant devices with resonance wavelengths in micrometer range, have gained significant interests due to its potential applications in integrated optics for optical telecommunication. Micro-resonators are useful as add-drop filters in wavelength division multiplexing (WDM) applications in optical telecommunication, since they can be designed to have resonance at the telecommunication wavelengths. In WDM applications, each micro-resonator adds or drops distinctive wavelengths of light that are resonant with the device. In such applications, an ability to locally tune the resonance of micro-resonators according to the specific wavelengths is crucial for successful implementation of micro-resonators in integrated optics.
Small micro-resonators, formed from high index difference (difference in the refractive indices of core and cladding) waveguide geometries are particularly useful since their free spectral ranges are large. High index difference waveguides, typically have index difference between the core and cladding equal to or larger than 0.3 and can be made in several different geometries, including channel waveguides and rib waveguides. A channel waveguide is a dielectric waveguide whose core is surrounded by a cladding that is composed of a material or materials with refractive indices lower than that of the core, and wherein the peak optical intensity resides in the core. High index difference waveguides can be defined in other waveguide geometries including a rib waveguide. A rib waveguide is a dielectric waveguide whose core is surrounded by a cladding that is composed of materials of which at least one has the same refractive index as that of the core. In waveguide configurations that are difference from a channel waveguide, a high index difference waveguide is defined as one that has a mode-field size similar to that of a high index difference channel waveguide (within 50% difference in cross-sectional area). In these waveguides, cladding is defined as a region where the evanescent field of optical modes exists.
Changing the characteristics of the resonance shape and position of a waveguide micro-resonator is an extremely important issue since the usefulness of such devices is predicated on such technology. One application of the waveguide micro-resonator is narrow band optical filtering in integrated optics. Wavelength division multiplexing (WDM), an increasingly used technology in optical communications, requires the use of such filters. Therefore, developing an efficient method of modifying the characteristics of such waveguide micro-resonators has been the subject of much research.
There are two approaches to changing the characteristics of the resonance shape. The first is to understand what characteristics of the response may be changed. For example, the resonance Q, or its quality, its position in the wavelength or frequency domain and its shape may all be changed.
The quality or the Q of the resonance can be changed by affecting the-amount of time the energy stays in the resonator. One method shown to affect the quality of the resonance includes inducing absorption in a micro-resonator and a method to affect the shape by using cascaded micro-resonators. This first method is difficult to implement, since the amount of absorption that has to be induced is large and the method cannot be easily applied to indirect-band-gap semiconductors and wide band gap dielectric materials. The second method, while useful, does not lend itself well to any dynamic changes in the resonance, which is necessary for switching or modulating or even tuning the resonance of the micro-resonator.
The resonance position, that is, the resonant wavelength or equivalently the resonant frequency of an optical micro-resonator is determined by the physical dimension of the device as well as the index of refraction of the materials that comprise the cavity. Changing the effective and group indices of the cavity mode can therefore change the resonant wavelength. Tuning of micro-ring micro-resonators by using a UV sensitive glass as a cladding material over the core of a low index contrast (typically a difference in index of core and cladding of less than 0.1) ring waveguide has also been shown. By changing the index of refraction of the cladding the effective and group indices of the mode of the ring waveguide changes, resulting in a shift in the resonance line position. While this method is effective for low index contrast waveguides, the method may be less effective for high index contrast (typically difference in index of core and cladding equal to or greater than 0.3) waveguides as the amount of index change required for high index contrast waveguides may be too large. However, small index changes in the cladding of high index contrast waveguides can lead to significant shifts in the line position sufficient for fine tuning applications.
Methods have also been shown to change the resonances of semiconductor micro-resonators by changing the refractive index of the core (guiding layer) of the micro-resonator. However, the methods do not include index changes in the cladding region and non-semiconducting substrates. Another method involves using the specific case of micro-ring filters with input and output waveguides that cross. Such a micro-ring filter configuration is necessarily a low index difference waveguide system because cross talk and losses are otherwise large in high index contrast systems.
Another method, which has been used extensively, is a thermo-optic tuning method in which the thermo-optic effect is used to change the index of the core of the micro-resonator cavity by a change in temperature. Thermal tuning, while simple and easy to implement has the disadvantage of significant cross talk in potential high density applications.
The second approach of analyzing how a resonance shape may be changed is to understand what physical aspects of the micro-resonator may be easily altered to have the desired effect on the characteristics of the resonance shape. For example, the absorption method and local proximity of multiple rings has been used to change the resonance shape of a micro-ring. Various other methods involve the change of the resonator internal rate of decay to change the resonance shape of micro-resonator devices. The internal rate of decay of resonator is determined by absorption and loss in the ring.
Another way to tune the resonance of a micro-resonator is to apply stress to shift its resonance positions. If the applied stress induces a change in the refractive indices of core and/or cladding materials, the resonance condition changes in the micro-resonators and the resonance peak will shift according to such a change.
Tuning of optical resonance by stress has been achieved previously. A method of tuning the resonance of a large optical resonator using a bonded piezo-electric element has been described. A piezo-electric element is bonded on the top surface of an optical resonator to supply stress to the underlying optical resonator when a voltage is applied to it. The stress applied to the resonator induces a change in the refractive index and thus changes the resonance. This method is applicable only for large, discreet optical element, and is not suitable for locally tuning resonance of micro-resonators, which are significantly smaller and typically integrated on-chip with waveguide input and output. Therefore, it is desired to have an ability to locally tune micro-resonators on-chip.
The thermo-optic effect and the use of the UV sensitive oxide, are examples of changing the resonance position by altering the effective and group indices of the modes in a micro-resonator cavity. In the invention, the focus is on other methods to change the position and shape and resonances of high index contrast waveguide micro-resonators, which are easier to implement.
The mechanisms to change the resonance of micro-cavity resonators are split along three lines in the literature according to the desired speed or equivalently, the time frame of their intended use. The fastest applications are in modulation, which usually occurs at the speed at which data is encoded. In communications, the speed is in excess of 1 GHz, which corresponds to times of less than 1 ns. Switching occurs at the speed at which data needs to be routed between lines in communications network. Slow switching is on the order of a ms, while packet switching may be as fast as 1 ns. Finally, tuning refers to permanent or long-term changes in the resonance.